1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly with transmitting and receiving radio frequency (RF) signals.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), radio frequency identification (RFID), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID reader, RFID tag, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system or a particular RF frequency for some systems) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Due to the substantially varying distances and/or orientation between a transmitter and receiver, the signal strength of the signals received by the receiver vary greatly (e.g., from 10 dBm to −90 dBm). In addition, RF signals typically experience multiple path fading (i.e., transmission of an RF signal to a receiver occurs over multiple paths that are of different lengths causing the signal strength to vary with minor changes in position). There are numerous solutions to these issues including transmit power adjustments, diversity antenna structures, multiple input multiple output (MIMO) transmission schemes, and beamforming.
As is known, a transmitter may adjust its transmit power levels according to the signal strength of the signals received by the receiver. If the signal strength is strong (e.g., above −10 dBm), the transmitter may reduce its transmit power level, thereby conserving energy and keeping the received signal within a certain signal strength level (e.g., −10 dBm to −50 dBm). If, on the other hand, the signal strength is weak (e.g., below −50 dBm), the transmitter may increase its transmit power level. Despite the adjustable transmit power levels, when the transmitter is transmitting at its maximum power level and the signal strength is weak, the receiver must still accurately recapture the information contained in the received RF signals.
As is also known, diversity antenna structures include two or more antennas that are space at one-quarter wavelength intervals. Each antenna receives the same RF signals and the received signal strength of each antenna is measured. The antenna having the strongest, or most consistently strong, signal strength is selected as the RF input for the receiver. This can be a dynamic process that changes as the receiver is moved.
MIMO transmission schemes includes two or more transmission and hence reception paths between a transmitter and receiver to communicate a single stream of information. Within the transmitter, the single stream of information is split into two or more baseband paths. Each baseband path is separately processed in accordance with a MIMO transmission matrix to produce a transmit RF signal. The transmission matrix provides a phase, frequency, and/or time relationship between the transmit RF signals such that, at the receiver, each baseband path can be accurately reproduced. The antennas of a MIMO transmission have the same linear polarization (i.e., omni-directional transmission).
To further improve MIMO wireless communications, the number of transmit antennas may exceed the number of receiver antennas such that the transceiver may incorporate beamforming. In general, beamforming is a processing technique to create a focused antenna beam by shifting a signal in time or in phase to provide gain of the signal in a desired direction and to attenuate the signal in other directions. In order for a transmitter to properly implement beamforming (i.e., determine a beamforming matrix), it needs to know properties of the channel over which the wireless communication is conveyed. Accordingly, the receiver must provide feedback information for the transmitter to determine the properties of the channel.
In satellite communication systems, multiple antennas are used to transmit and receive signals with a satellite. Since the transmission path between a satellite transceiver and a terrestrial transceiver is relatively fixed in distance and direction when compared to terrestrial wireless communications, satellite systems may use a different transmission scheme that terrestrial wireless communication systems. For instance, a satellite system may use circular polarization of opposite directions for transmitting and receiving signals. Due to the differences between satellite systems and terrestrial wireless systems, different transmission schemes are used.
Therefore, a need exists for a terrestrial wireless transmission scheme and/or antenna structure that provides improved directional wireless communications.